A conventional receiver is usually added with external components to receive different kinds of signals effectively. With reference to FIG. 1 to illustrate a super heterodyne receiver 1. The super heterodyne receiver 1 includes a low noise amplifier 11, a mixer 13, a voltage-controlled oscillator 14, and a demodulator 16, and the super heterodyne receiver 1 is added with an image rejection filter 12 and an IF filter 15. Thus, the super heterodyne receiver 1 needs large circuit area and large power dissipation.
To overcome the shortcomings of large circuit area and power dissipation, the method of amplitude-shift keying (ASK) is utilized with envelop detection to lower circuit area and power dissipation. For example, “Direct-conversion CMOS transceiver with automatic frequency control for 802.11a wireless LANs” published on International Solid-State Circuits Conference, 2003, described the transceiver including an inductor disposed on a chip to serve as a first-stage passive loaded circuit, and an inductor disposed outside the chip to match with an antenna. Besides, the transceiver needed to be accomplished with baseband demodulation circuits, thereby occupying circuit areas about 2 mm×1 mm. Consequently, the transceiver still consumed quite large circuit areas, and it is necessary to be improved for applications in the fields of wireless bio-diagnosis monitoring network and personal health monitoring system.
Furthermore, though the demodulator circuits have the advantage of low power dissipation, conventional demodulator circuits of amplitude modulation (AM) and amplitude-shift keying (ASK) result in very low transmission rate for the effect of clipping. The effect of clipping is caused by a big RC time constant, and the RC time constant makes the signal waveform partially clipped. As illustrated in FIG. 2, the waveform is turned off by a diode while the packet waveform falling faster than RC time constant frequency. Accordingly, the fall exponential waveform A is different from the original waveform, and the required packet signals are broken. Therefore, how to improve the shortcoming by circuit design is an important issue of the technology.